There are many active studies on mechanisms and therapeutic agents for various diseases due to immune disorders or immune depression caused by carcinostatic drugs. It is known that thymulin, a nonapeptide produced in the thymus, forms a complex with zinc and improves a depressed immunity (Med. Oncol. & Tumor Pharmacother. 6, 25-29, 1989).
However, there remain many problems in practical user, of thymulin. For example, the availability of thymulin is limited due to a small yield in the thymus and duration of the activity thereof is short because thymulin is easily decomposed by endogenous enzymes.
Therefore, synthetic compounds are desired which have a long lasting activity and can be prepared in a large amount. As such synthetic compounds, lactone or lactam compounds having two intermolecular sulfur atoms in the side chain are proposed (PCT/JP92/00002). As shown in the PCT application, the lactone or lactam compounds show excellent thymulin-like activities.
However, it is further desired to study the derivatives thereof to find another useful compounds.
The inventors intended to convert the lactone or lactam compounds disclosed in PCT/JP92/00002 into bicyclic compounds and succeeded in obtaining novel bicyclic compounds which have excellent thymulin-like activities.